It will use the Autopilot cameras (there are no additional sensors), and the Tesla Vision software on AP2 hardware does not support detection of rain on the windscreen. The driver has to turn the wipers on manually. My Renault Zoe, which costs about 25% of my Model X, does this automatically.

Tesla Autopilot is cool, but the software is very limited at present. The original link is essentially what Tesla think they can achieve with the AP2 hardware, but the production software is nowhere near capable of these feats.